Allow echidna-parade to stop an onging fuzzing campaign and save the best corpus

[agroce]

Ideally, we could have a special echidna-parade command to stop a certain campaign:

echdina-parade --stop parade.XXXXX This will terminate all the corresponding echidna instances and the actual script, but saving the final corpus in a particular directory, like parade.XXXXX/final

My comment was:

I think if you just kill, the current best corpus from all generations but latest will be saved anyway, BTW.